Fiber 101: Soluble Fiber vs Insoluble Fiber

Written By Gloria Tsang, RD on Jan 29, 2013

(HealthCastle.com) We all know the benefits of fiber! Fiber not only promotes gut health, but also helps reduce the risk of developing many chronic diseases. For instance, fiber prevents constipation, hemorrhoids and diverticulosis. A high-fiber diet is also linked to lower risk of developing some cancers, especially colon and breast cancer. In addition, it helps lower LDL cholesterol and total cholesterol. Furthermore, high-fiber foods generally have a lower glycemic index value, an important element in managing Type 2 diabetes.

Types of Fiber: Soluble Fiber and Insoluble Fiber

Both soluble and insoluble fiber are undigested. They are therefore not absorbed into the bloodstream. Fiber content is often listed under "Total Carbohydrates" on a Nutrition Facts label.  Because it is undigested, it provides 0 calories. Instead of being used for energy, fiber is excreted from our bodies. Soluble fiber forms a gel when mixed with liquid, while insoluble fiber passes through our intestines largely intact.

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Insoluble Fiber

Functions of Insoluble Fiber

• move bulk through the intestines
• control and balance the pH (acidity) in the intestines

Benefits of Insoluble Fiber

• promote regular bowel movement and prevent constipation
• remove toxic waste through colon in less time
• help prevent colon cancer by keeping an optimal pH in intestines to prevent microbes from producing cancerous substances

Top Insoluble Fiber Foods

• Wheat bran, 11.3 grams of insoluble fiber per 1/2 cup
• All Bran cereal, 7.2 g per 1/3 cup
• Most beans (1/2 cup)
  • Kidney beans, 5.9 g 
  • Pinto beans, 5.7 g 
  • Navy beans, 4.3 g
• Lentils, 4.6 g per 1/2 cup
• Shredded Wheat cereal, 4.5 g per cup
• Most Whole grains. Bulgur, for instance, contains 4.2 grams of insoluble fiber in 1/2 cup
• Flax seeds, 2.2 g per 1 tbsp
• Vegetables (1/2 cup)
  • Okra, 3.1 g 
  • Turnip, 3.1 g
  • Peas, 3 g

Soluble Fiber

Functions of Soluble Fiber

• bind with fatty acids
• prolong stomach emptying time so that sugar is released and absorbed more slowly

Benefits of Soluble Fiber

• lower total cholesterol and LDL cholesterol (the Bad cholesterol) therefore reducing the risk of heart disease
• regulate blood sugar for people with diabetes

Top Soluble Fiber Foods

• Purple passion fruit, 6.5 g of soluble fiber per 1/2 cup
• Psyllium husk, 3.5 g per 1 Tbsp
• Metamucil, 3.4 g per 1 Tbsp
• Oat/Oat bran, 2.2 g per 3/4 cup
• Some Beans (1/2 cup)
  • Black beans, 2.4 g
  • Navy beans, 2.2 g
  • Kideny beans, 2 g
• Soy
  • Tofu, 2.8 g per 3/4 cup
  • Edamame, 1.5 g per 1/2 cup
• Vegetables (1/2 cup)
  • Avocado, 2.1 g
  • Brussels sprouts, 2 g
  • Sweet potato, 1.8 g
  • Asparagus, 1.7 g
  • Turnip, 1.7 g
• Fruit
  • Dried figs, 1.9 g per 1/4 cup
  • Orange, 1.8 g, medium size
  • Fruit with skin, like pear, apricots, and nectarine, ~ 1.2 g
• Flax seed, 1.1 g per 1 tbsp

The Bottom Line

An average diet contains 75:25 insoluble fiber: soluble fiber ratio. When making a food choice decision, don't worry about choosing a specific type of fiber. Many foods such as oat, oat brans, psyllium husk and flax seed are rich in both insoluble and soluble fiber. Eating enough fiber is more important! The recommended intake of fiber is 25 g per day. If you eat at least 5 servings of fruits & vegetables as well as at least 6 servings of grain products per day (at least 3 of which are whole grains), you are very likely meeting the fiber requirements.

* Fiber Content of Foods in Common Portions by Harvard University Health Services, dated May 2004, and, Food Sources of Fibre by Dietitians of Canada, dated Jul 27, 2012.